JNI: don't access null jobject

When Java calls a native function that expects a QString with a null
String object, then we get a nullptr jstring. We must not try to convert
that to a QString.

Add the necessary checks at the call-site of the toQString helper, and
an assert to the helper itself. Add test case.

Change-Id: If5c55c702bdb3f89ac45fdf3912af2ac295f5e5c
Reviewed-by: Volker Krause <vkrause@kde.org>
(cherry picked from commit db6a9834d2b109e81cc73ef3c64d0c420237e9dc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qjniarray.h

@@ -819,12 +819,14 @@ public:
             container.reserve(sz);
         if constexpr (std::is_same_v<typename ContainerType::value_type, QString>) {
             for (auto element : *this) {
-                if constexpr (std::is_same_v<decltype(element), QString>)
+                if constexpr (std::is_same_v<decltype(element), QString>) {
                     container.emplace_back(element);
-                else if constexpr (std::is_same_v<decltype(element), jstring>)
-                    container.emplace_back(QtJniTypes::Detail::toQString(element, env));
-                else
+                } else if constexpr (std::is_same_v<decltype(element), jstring>) {
+                    container.emplace_back(element ? QtJniTypes::Detail::toQString(element, env)
+                                                   : QString{});
+                } else {
                     container.emplace_back(QJniObject(element).toString());
+                }
             }
         } else if constexpr (std::is_base_of_v<std::remove_pointer_t<jobject>, std::remove_pointer_t<T>>) {
             for (auto element : *this)

src/corelib/kernel/qjnitypes.h

@@ -163,7 +163,7 @@ struct JNITypeForArgImpl<QString>
 
     static QString fromVarArg(Type t)
     {
-        return QtJniTypes::Detail::toQString(t, QJniEnvironment::getJniEnv());
+        return t ? QtJniTypes::Detail::toQString(t, QJniEnvironment::getJniEnv()) : QString();
     }
 };
 
@@ -190,7 +190,7 @@ public:
 
     static QList<T> fromVarArg(Type t)
     {
-        return QJniArray<ElementType>(t).toContainer();
+        return t ? QJniArray<ElementType>(t).toContainer() : QList<T>{};
     }
 };
 

src/corelib/kernel/qjnitypes_impl.h

@@ -24,6 +24,7 @@ static inline jstring fromQString(const QString &string, JNIEnv *env)
 
 static inline QString toQString(jstring string, JNIEnv *env)
 {
+    Q_ASSERT(string);
     const jsize length = env->GetStringLength(string);
     QString res(length, Qt::Uninitialized);
     env->GetStringRegion(string, 0, length, reinterpret_cast<jchar *>(res.data()));

tests/auto/corelib/kernel/qjniobject/testdata/src/org/qtproject/qt/android/testdata/QtJniObjectTestClass.java

@@ -307,6 +307,7 @@ public class QtJniObjectTestClass
     native public int callbackWithRawArray(Object[] value);
     native public int callbackWithQList(double[] value);
     native public int callbackWithStringList(String[] value);
+    native public int callbackWithNull(String str);
 
     public int callMeBackWithObject(QtJniObjectTestClass that)
     {
@@ -358,6 +359,10 @@ public class QtJniObjectTestClass
     {
         return callbackWithStringList(value);
     }
+    public int callMeBackWithNull()
+    {
+        return callbackWithNull(null);
+    }
 
     public Object callMethodThrowsException() throws Exception {
         throw new Exception();

tests/auto/corelib/kernel/qjniobject/tst_qjniobject.cpp

@@ -1959,6 +1959,7 @@ enum class CallbackParameterType
     RawArray,
     QList,
     QStringList,
+    Null,
 };
 
 static std::optional<TestClass> calledWithObject;
@@ -2047,6 +2048,14 @@ static int callbackWithStringList(JNIEnv *, jobject, const QStringList &value)
 }
 Q_DECLARE_JNI_NATIVE_METHOD(callbackWithStringList)
 
+static std::optional<QString> calledWithNull;
+static int callbackWithNull(JNIEnv *, jobject, const QString &str)
+{
+    calledWithNull.emplace(str);
+    return int(CallbackParameterType::Null);
+}
+Q_DECLARE_JNI_NATIVE_METHOD(callbackWithNull)
+
 void tst_QJniObject::callback_data()
 {
     QTest::addColumn<CallbackParameterType>("parameterType");
@@ -2062,6 +2071,7 @@ void tst_QJniObject::callback_data()
     QTest::addRow("RawArray")   << CallbackParameterType::RawArray;
     QTest::addRow("QList")      << CallbackParameterType::QList;
     QTest::addRow("QStringList") << CallbackParameterType::QStringList;
+    QTest::addRow("Null")       << CallbackParameterType::Null;
 }
 
 void tst_QJniObject::callback()
@@ -2171,6 +2181,15 @@ void tst_QJniObject::callback()
         QCOMPARE(calledWithStringList.value(), strings);
         break;
     }
+    case CallbackParameterType::Null: {
+        QVERIFY(TestClass::registerNativeMethods({
+            Q_JNI_NATIVE_METHOD(callbackWithNull)
+        }));
+        result = testObject.callMethod<int>("callMeBackWithNull");
+        QVERIFY(calledWithNull);
+        QCOMPARE(calledWithNull.value(), QString());
+        break;
+    }
     }
     QCOMPARE(result, int(parameterType));
 }